In recent years, X-ray CT apparatuses that perform a photon counting Computed Tomography (CT) by employing a photon-counting type detector have been developed. Unlike integral-type detectors used in conventional X-ray CT apparatuses, the photon-counting type detector outputs signals that make it possible to individually count photons derived from X-rays that have passed through an examined subject (hereinafter, a “subject”). Accordingly, by performing the photon counting CT, it is possible to reconstruct an X-ray CT image having a high Signal-per-Noise (S/N) ratio.
Further, the signals output by the photon-counting type detector can be used for measuring (discriminating) an energy level of each of the counted photons. Accordingly, by performing the photon counting CT, it is possible to image data acquired by radiating X-rays while using one type of X-ray tube voltage in such a manner that the data is divided into a plurality of energy components. For example, by performing the photon counting CT, it is possible to generate an image that makes it possible to identify one or more substances by utilizing differences in K absorption edges. Examples of the substances include contrast agents and chemical labeling substances that are able to specifically label tissues.
As explained here, by performing the photon counting CT, it is possible to generate the image that makes it possible to identify the substances of interest, by discriminating the individual photons so as to be divided into the energy components. However, if the energy discrimination is finely performed, the amount of data becomes extremely large, and it takes a long time to transfer data and perform the reconstruction. On the contrary, if the energy discrimination is roughly performed, it is not possible to obtain the data necessary for identifying the substances of interest.